This invention relates to a lower limb prosthesis including a dynamically adjustable joint movement control unit arranged to control either or both of flexion and extension of the joint automatically.
It is known from British Patent Application No. 2280609A to provide a lower limb prosthesis with a dynamically adjusting control system for controlling the movement of a shin part of the prosthesis about a knee axis on a thigh part of the prosthesis according to the amputee""s speed of locomotion. A pneumatic piston and cylinder device coupled between the thigh part and the shin part has a motor-driven valve which alters the resistance of the device to movement at the knee joint in response to command signals from an electronic control circuit deriving input signals from a transducer mounted on the control device, the repetition rate of the input signals being representative of the speed of locomotion, particularly the step period during walking.
The control circuit includes a radio receiver for receiving command signals from a remote control transmitter operated by a prosthetist, a processor for processing the command signals and the transducer signals, and a memory for storing a map of valve settings against locomotion speed ranges. The processor has a teaching mode whereby the amputee is asked to walk at a particular speed and the system is xe2x80x9ctaughtxe2x80x9d by the prosthetist inasmuch as the prosthetist causes the valve to be adjusted under remote control in real time while the amputee is walking until the best gait is obtained. The same process is performed at different walking speeds and the prosthetist selects a valve setting for each speed which, in his or her opinion, appears to produce the best walking gait. These valve settings are stored at the end of a teaching session. In a playback mode of the processor, signals corresponding to the stored valve settings are fed to the motor-driven valve automatically according to the speed at which the amputee walks.
The above system has yielded notable improvements in gait for above-knee amputees due to its adaptation of resistance to knee joint movement to different settings suiting different walking speeds rather than relying on a fixed resistance setting for all walking speeds. These improvements have been achieved without requiring excessively lengthy sessions with the prosthetist.
The field of gait analysis in general has received considerable attention over the years. In xe2x80x9cRepeatability of Kinetic and Kinematic Measurements in Gait Studies of the Lower Limb Amputeexe2x80x9d by Zahedi et al in Prosthetics and Orthotics International, 1987, 11, 55-64, a computational method is disclosed for performing gait measurements which are useful in biomechanical evaluation. One of the observations arising out of this work is that small differences in the geometric alignment of a lower prosthesis influence the degree of repeatability and pattern of load actions. It is also suggested that, from biomechanical considerations, alignments which have the least variation in loads from step to step in antero-posterior bending moment and axial loading parameters may be nearer to an optimum condition.
According to a first aspect of this invention, there is provided a lower limb prosthesis which automatically reacts to a variability measurement dynamically to adjust a control device which forms part of the prosthesis and which affects the flexion and/or the extension of a joint connecting different parts of the prosthesis. In this way, it is possible to provide a self-learning adaptive control system for a lower limb prosthesis, the system measuring the variation of one or more parameters associated with the dynamic operation of the limb, and automatically processing the variation measurement to optimize or reduce the variability of the parameter, preferably using an iterative process, in order to achieve an optimum locomotion characteristic.
The system is primarily applicable to a lower limb prosthesis for an above-knee amputee, the control device being a knee flexion control device such as a piston and cylinder assembly having an electrically adjustable valve responsive to an adjusting signal generated by a microprocessor which is programmed to derive a kinematic parameter variability value from input signals produced by a transducer mounted on the limb.
The kinematic parameter may be the amplitude of the flexion angle of the joint which, in the case of flexion and/or extension of the joint being controlled by a piston and cylinder device connected between a side part of a shin part of the prosthesis, may be represented by the amplitude or magnitude of the piston stroke. Alternatively or in addition, the duration of the flexed state (e.g. the time interval from commencement of the flexion movement to termination of the extension movement, these points typically being determined by setting angular thresholds or piston positional thresholds) may be used as another kinematic parameter, preferably as a proportion of the total step period. As a further alternative, stride length may be used as a kinematic parameter.
According to a second aspect of the invention, there is provided a lower limb prosthesis for an above-knee amputee, the prosthesis including a dynamically adjustable knee movement control unit arranged to control flexion and/or extension of a knee joint of the prosthesis automatically in response to the sensed step to step variability of at least one kinetic or kinematic parameter of locomotion in order to reduce the said step to step variability. Typically, the sensed variability is an electrical signal value representative of the degree of variation of a kinematic parameter measured during each of a plurality of steps taken by the amputee during locomotion, the parameter being measured during each step taken by the amputee which is within a predetermined range of locomotion, such as a particular walking speed range or a particular category of locomotion. In this context xe2x80x9ccategory of locomotionxe2x80x9d means different modes of locomotion such as walking on a level surface, walking down an incline, walking up an incline, walking down stairs, climbing up stairs, or running. Speeds of walking or running as speed ranges may be determined by measuring the repetition rate or the average step period of a walking or running cycle, each cycle extending, for instance, from heel contact to heel contact through stance phase and swing phase.
The control system may be configured to determine the variability of one or more kinetic or kinematic parameters over each of a plurality of the ranges of locomotion so that the control device is adjusted to a plurality of optimum settings for the different respective locomotion ranges. It is possible, then, for the system to determine the range of locomotion from received electrical signals from one or more transducers forming part of the prosthesis, and, using the same signals, to perform an iterative variability measurement and adjustment process within each respective locomotion range. The process minimizes variability of the selected parameter or, in the case of the variability of a plurality of parameters being measured, minimizes the variability of at least one of them (which is designated the primary parameter). Generally, altering or minimizing the variability of a kinematic parameter is associated with altering or minimizing an underlying kinetic parameter of locomotion.
According to a third aspect of the invention, a lower limb prosthesis includes a dynamically adjustable control device for controlling movement of the prosthesis during locomotion, a transducer for generating a sensing signal related to a kinematic parameter of locomotion, and an electronic control circuit having an input coupled to the transducer and an output coupled to the control device, wherein the transducer and the control circuit are configured to determine the variability of the kinematic parameter and to generate output signals for adjusting the control device thereby to reduce the variability of the parameter. In the preferred prosthesis, the control circuit is configured to record a value of the kinematic parameter during each of a plurality of locomotion cycles, to compare the recorded values to establish a variability measurement, to compare the variability measurement with a reference variability value and, if the variability measurement exceeds the reference value, to initiate a control device adjustment procedure in which the control device is dynamically adjusted so as substantially to minimize the variability of the or each parameter.
From a method aspect, the preferred control system measures the speed of walking, computes the variation of a kinematic parameter over a number of steps, processes the measured parameter data in order to determine whether the degree of variation falls within a band of optimum parameter variation, adjusts the resistance to joint flexion and/or extension by a predetermined increment in accordance with the degree of variation of the parameter or parameters in order to reduce the amount of variability. The corresponding control device settings, in conjunction with walking speed values, can be stored in order that, in a playback mode, the control device is adjusted to a variability-minimizing setting corresponding to a measured walking speed as determined by the stored relationships.
The optimization process may be carried out continuously during use of the prosthesis, variation of the kinematic parameters being iteratively reduced whenever the variability deviates from a predetermined optimum condition. The speed of walking may be defined according to a number of non-overlapping speed ranges which might be designated xe2x80x9cslowxe2x80x9d, xe2x80x9cmediumxe2x80x9d, and xe2x80x9cfastxe2x80x9d, the stored data associating a limb control device setting for each range which has been determined by means of the automatic self-learning process as yielding substantially the minimum variation in the kinematic parameter or parameters.